Their mother’s love carries on. In a preview for an upcoming documentary titled Diana, Our Mother: Her Life and Legacy, Princess Diana’s beloved sons, Prince William and Prince Harry, come together to honor their late mom’s legacy while looking through a family album.

Princess Diana, Prince William and Prince Harry pose for photos at the piano in Kensington Palace on October 4, 1985.Tim Graham/Getty Images

The documentary marks the first time the royal brothers have joined together to publicly express their thoughts on the late Princess of Wales as a parent.

Recalling fond memories, William — who is father of Prince George, 3, and Princess Charlotte, 2, with Duchess Kate — looks at a photo of himself as a child with a glowing Diana and tells his brother, “Believe it or not, you and I are both in this photograph. You’re in the tummy.”

Princess Diana, Prince William and Prince Harry pose for photos at the Grounds of Highgrove in Tetbry, Gloucestershire on July 18, 1986.Tim Graham/Getty Images

“This is the first time the two of us have ever spoken about her as a mother. She was very informal and really enjoyed the laughter and the fun,” William reveals in the clip. “She was one of the naughtiest parents — but she understood that there was a real life outside of palace walls.”

Personal family shots — including a photograph of Diana walking alongside Harry as William rides a pony, plus a video clip of her extending open arms to welcome her sons — highlight the adoration the royal mother had for her sons.

“She was our mum. She still is our mum. And of course, as a son I would say this, she was the best mum in the world,” Harry says. “She smothered us with love, that’s for sure.”

Prince Harry’s Hottest Moments! Prince William and Prince Harry cheer on runners at the 2017 Virgin Money London Marathon in London, England on April 23, 2017.Max Mumby/Indigo/Getty Images

On August 31, 1997, Diana was killed in a car crash in Paris. Her death gripped the nation and highlighted the often intrusive nature of paparazzi, whom many blamed for her death as it was widely reported that the car Diana had been in was speeding to escape photographers.

The documentary comes on the heels of the 20-year anniversary of Diana’s death.

Diana, Our Mother: Her Life and Legacy will air on HBO later this month.